In engine fuel delivery systems of current design, fuel is fed by a constant-delivery pump from a fuel tank to the engine, and excess fuel is returned from the engine to the fuel tank. Such return fuel carries engine heat to the fuel supply, and consequently increases temperature and vapor pressure at the fuel supply. Venting of excess vapor pressure to the atmosphere not only causes pollution problems, but also deleteriously affects fuel mileage. Excess fuel tank temperature can also cause vapor lock at the pump, particularly where fuel level is relatively low. Constant pump operation also increases energy consumption while decreasing both pump life and fuel filter life.
U.S. Pat. No. 4,649,884 discloses a fuel injection system for an internal combustion engine in which an electric-motor constant-delivery fuel pump supplies fuel under pressure from a tank to a fuel rail positioned on the engine. Excess fuel is returned to the supply tank as a function of pressure differential between the fuel rail and the engine air intake manifold. A plurality of fuel injectors are mounted between the fuel rail and the engine air manifold, with the injector nozzles being positioned adjacent to the fuel/air intake ports of the individual engine cylinders. U.S. application Ser. No. 126,517, filed Nov. 30, 1987 and assigned to the assignee hereof, now U.S. Pat. No. 4,789,308 discloses a fuel delivery system for an internal combustion engine in which outlet pressure of an electric-motor fuel pump is monitored, and pump motor current is controlled as a function of such outlet pressure. Although the fuel delivery systems disclosed in the noted patent and application address the aforementioned problems in current fuel delivery system designs, further improvements remain desirable.
An object of the present invention, therefore, is to provide a fuel delivery system, particularly a fuel injection system of the type disclosed in the above-noted patent, that maintains constant pressure differential across the fuel delivery mechanism, specifically the fuel injectors, so that quantity of fuel supplied for a given injector activation time remains substantially constant and independent of fluctuations in air manifold pressure. Another object of the invention is to provide a pressure differential control system of the described character that is economical to implement in mass production of automotive fuel delivery systems, for example, and is reliable over an extended vehicle lifetime. A further object of the present invention is to provide a fuel delivery system of the described character that achieves on-demand fuel delivery, and thus reduces energy consumption while increasing pump and fuel filter operating lifetimes. Yet another object of the invention is to provide a fuel delivery system of the described character that reduces delivery of engine heat to the fuel tank, and thus reduces problems associated with fuel vaporization as hereinabove discussed. A further object of the invention is to provide a fuel delivery system that implements electronic control of the fuel pump as a function of fuel requirements, and in which the control electronics is cooled by fuel circulating in the delivery system.